On June 2005, a respiratory disease was observed in the chickens of a large broiler farm in Kermanshahprovince, west of Iran. Relatively severe respiratory signs started with sneezing at 27 days of age. Thedisease lasted up to the end of fattening period and accompanied by increased mortality (13.6%). At postmortemexamination, tracheitis, airsacculitis and pneumonia were obvious. Serologic examinations werenegative for Mycoplasma gallisepticum and Mycoplasma synoviae. On virologic examinations, virulentinfectious bronchitis virus (IBV), avian influenza virus (AIV) and virulent Newcastle disease virus (NDV)could not be isolated. Histopathologic examinations showed no pathognomonic lesion typical for infectiouslaryngotracheitis. On bacteriologic examinations, Ornithobacterium rhinotracheale (ORT) was isolated fromtrachea, lungs and air sacs of the affected birds. Based on clinical, post-mortem and laboratory findings, ORTcould be probably the primary cause of respiratory disease on this farm